The present invention relates to the high pressure/high temperature (HP/HT) process for making polycrystalline diamond or cubic boron nitride cutting inserts. More particularly, the invention relates to certain modifications in the HP/HT process which results in cutting inserts with improved production economy and clamping possibilities.
For machining of several non-ferrous alloys like brass, magnesium and aluminum, diamond is the best tool material whereas cubic boron nitride (cBN) is very well suited for machining hardened steel, chilled cast iron and cast iron at elevated speeds.
The inserts used in such operations are either so-called PCD (polycrystalline diamond compact) or PcBN (polycrystalline cubic boron nitride compact) comprising a cemented carbide body with a diamond or cBN layer applied at such high temperature and pressure where diamond or cBN is the stable phase or the inserts are provided with diamond or cBN bodies in at least one corner or along an edge generally fastened by brazing.
The method for manufacturing such inserts has been described in, e.g., U.S. Pat. Nos. 3,745,623, 3,743,489, 4,525,179, 5,598,621 and 5,676,496.
Many inserts used for machining have holes in the center to ensure the best possible clamping to the tool holder. In case of brazed inserts, this can be achieved by brazing a PCD/cBN tip to a body that already has a hole in it. Otherwise, the hole has to be cut afterwards which is costly since cutting usually involves electro discharge machining (EDM).
U.S. patent application Ser. No. 08/601,631 discloses a method of making a cutting insert with a hole for clamping to a tool holder wherein a superhard abrasive material is sintered and simultaneously bonded to a sintered cemented carbide body with a hole inside a container under elevated pressure and temperature conditions. During sintering, the hole is filled with a plug which after sintering is removed. The plug may consist of a refractory metal, cemented carbide or cemented carbide surrounded by a sleeve of a refractory metal. The refractory metal is preferably niobium. The disadvantage with this method is that the refractory metal cannot withstand the high pressure and temperature resulting in cracks in the cemented carbide insert.